Ring-opening polymerization is the most efficient way of obtaining aliphatic (co)polyesters from monomers selected from the group comprising lactides, of L- or D-configuration, lactones, cyclic carbonates and cyclic anhydrides. These synthetic polymers are of considerable interest as biodegradable materials.
In view of their intrinsic properties, biodegradable polymers have become an important alternative to synthetic polymers based on compounds obtained from petrochemistry, and much progress has been made both from the standpoint of synthesis and of processing of these materials. Moreover, they are used for a large number of applications, such as packaging and textiles. Among the various biodegradable polymers, polylactide (PLA) is one of the most commonly used and studied.
The ring-opening polymerization of lactide is the route of synthesis commonly used for the production of polylactide. It is generally carried out as solution polymerization or bulk polymerization (in the absence of solvent) in the presence of a catalytic system, and sometimes even in the presence of an initiator of the alcohol or amine type. The catalysts used are described in the literature and notably in patent EP 0 615 532, for example Sn(II) bis(2-ethylhexanoate), butyltin tris(2-ethylhexanoate) and dibutyltin diacetate. We may also mention the use of Lewis bases as co-catalyst, as described in patent U.S. Pat. No. 6,166,169, and notably the catalytic system tin octanoate (Sn(Oct)2)/triphenyl phosphine (P(Ph)3) commonly used at present in the polymerization of lactide.
There is, however, a desire to perform ring-opening polymerization of lactide in the presence of tin-free catalysts.
There is also a desire to perform ring-opening polymerization of lactide as a bulk polymerization. In fact, bulk polymerization is particularly suited to industrial application as it provides rapid production of polylactide and direct use of the polymer, in contrast to solution polymerization, after which the polymer obtained must be separated from the solvent, which on the one hand increases the complexity of the process and on the other hand affects its economic effectiveness.
Document U.S. Pat. No. 7,169,729 discloses the solution oligomerization of D,L-lactide at 40° C. in the presence of the catalyst Zn[N(SiMe3)2]2.
Document U.S. Pat. No. 6,297,350 discloses the bulk polymerization of L-lactide at 150° C. in the presence of alcohol and of zinc lactate as catalyst. However, the yield is still relatively low.
Document US 2007/0083019 discloses the ring-opening polymerization of oligomers of cyclic ω-hydroxycarboxylic acids in the presence of a zinc-based organometallic complex. This document does not, however, disclose the polymerization of lactide in the presence of this complex.
J. Am. Chem. Soc., 2003, 125, 11350-11359 discloses the solution polymerization of lactide at room temperature in the presence of a zinc-based organometallic complex. This document does not, however, disclose the high-temperature bulk polymerization of lactide with this complex.